A core bit employed for boring a concrete structure or the like has a cylindrical tube and diamond tips fixed to an opening end surface of the tube at regular intervals in the circumferential direction. The core bit is rotated and driven by a motor or the like for pressing the opening end surface of the tube against a surface of the concrete structure or the like, thereby boring the concrete structure or the like while cutting an annular groove in the surface thereof.
Such a core bit is classified into a wet-type core bit employing cooling water in a perforating operation and a dry-type core bit with air cooling airflow for performing cooling by feeding an airflow.
The wet-type core bit is capable of cutting with a heavy load, and hence has high productivity. When employing the wet-type core bit, however, there arise such a problem that the consumption of energy is high and such an environmental problem that the cooling water contaminates the workpiece. In the dry-type core bit, on the other hand, the environmental problem of contaminating the workpiece is small since no cooling water is employed. When employing the dry-type core bit, however, end surfaces of the diamond tips involved in cutting are heated to a high temperature, and hence the dry-type core bit has such a disadvantage that the tool life thereof is short or the like.
However, since an operation for boring a concrete structure such as a building material is generally performed at the so-called work site in a place where a building structure is present and hence it is difficult to secure cooling water, the convenient dry-type core bit is mainly employed.
FIG. 1 is a partial sectional view showing a perforation apparatus including a core bit and a concrete structure selected as a workpiece in a perforating operation employing a dry-type core bit. As shown in FIG. 1, the core bit has a tube 2, a flange 1 fixed to one end of the tube 2, and a plurality of tips 3 fixed to the other end of the tube 2. The plurality of tips 3 are fixed to an opening end surface of the tube 2 at regular intervals along the circumferential direction. A perforation apparatus 10 is mounted on the flange 1 of the core bit. The perforation apparatus 10 has an axial hole 5 so that compressed air circulates therethrough as shown by arrows. Forward end surfaces of the tips 3 are pressed against a surface 41 of the concrete structure 4 while rotating and driving the core bit with the perforation apparatus 10. Thus, a perforating operation is performed to form an annular groove 42 in the concrete structure 4. At this time, the compressed air is introduced into the tube 2 through the axial hole 5, passes through the annular groove 42, reaches the outer side of the tube 2 through the forward end surfaces of the tips 3, and passes through the annular groove 42 again to be effused into the air, as shown by arrows. Cooling of the tips 3 and discharge of chips resulting from cutting of the concrete structure 4 are performed by this airflow.
Each one of the tips 3 consists of an abrasive grain layer. The abrasive grain layer is formed by diamond abrasive grains and a metal bond serving as a binder for bonding the diamond abrasive grains to each other. The metal bond is mainly composed of hard grains of tungsten or the like and cobalt. Tips structured in such a way are frequently employed in general, and are of a type of diamond abrasive grains mixed into a metal bond. A core bit having tips of such a type is called an impregnated bit. When employing a core bit of this type, an autogenous action successively provides new surfaces of diamond abrasive grains during the perforation operation as wear of the diamond tips progresses by chips.
In case of performing a boring operation in a dry type with an impregnated bit, forward end surfaces of tips are strongly pressed against a surface of a concrete structure. In this case, the amount of heat generated by the friction between the tips and the concrete structure is significant in contrast to the case of the wet type operation employing cooling water. Further, a cylindrical groove formed by cutting the concrete structure is narrow and small, and hence compressed air cannot smoothly flow in the groove. Thus, a cooling effect with the compressed air is weak, and hence parts of the diamond tips concerned in cutting are heated to a high temperature.
The diamond abrasive grains start to be thermally damaged when heated to at least 600.degree. C. in the air. When the diamond abrasive grains are heated to at least 900.degree. C., further, the diamond abrasive grains are gasified, crushed or worn before the metal bond is worn. Consequently, the autogenous action of the diamond abrasive grains is inhibited in the perforating operation, and the core bit cannot perform cutting. In case of employing the conventional core bit, therefore, it has been impossible to increase the perforating speed by strongly pressing the forward end surfaces of the diamond tips against a surface of a workpiece.
In order to enable the autogenous action of the diamond abrasive grains to continuously take place, therefore, an easily worn substance may be employed as the material for the metal bond. However, there has been such a problem that the diamond tip itself becomes fragile and the strength lowers when employing an easily worn substance as the material for the metal bond.
As another means for enabling the autogenous action of the diamond abrasive grains to continuously take place, the tip may be reduced in size for reducing the number of the diamond abrasive grains, in order to increase a load applied to the diamond abrasive grains. When employing this means, however, there have been such problems that not only does the strength of the tip decrease, but also vibration increases in a perforating operation particularly while cutting a reinforcing bar or the like. As a result, diamond abrasive grains crush or fall at an increased rate, and the tip is worn in an early stage.
An object of the present invention is to provide a core bit having a high cutting speed, i.e., excellent sharpness, being excellent in durability, and having a long life.